Large quantities of dilute waste waters resulting from a variety of industrial operations (e.g., electroplating) can often be electrochemically treated to detoxify them or to recover valuable metals therefrom before discharge to the environment. Likewise, electrolytic purification processes may be used to provide substantially metal-free process waters needed for a variety of industrial applications. However, reactors built for the electrochemical scavenging and deposition of dissolved metals from dilute, aqueous solutions thereof tend to suffer from poor electrical conductivity, low reaction (i.e., metal deposition) rates, poor metal deposition current efficiency, and low quality metal deposits. Poor electrical conductivity results from too few ions to carry current passed through the solution. Poor metal deposition current efficiency results from a substantial proportion of the current being wastefully consumed on alternate and competitive reactions (e.g., hydrogen gassing) which occur at the cathode simultaneously with metal deposition. Low metal deposition rates result from limited mass transport of metal ions to the cathode (i.e., poor diffusional and electrical transport of metal ions through the solution). Low quality (e.g., impure, dendritic, etc.) deposits result both from the limited ability of the dilute aqueous solution to supply metal ions to the cathode, and from the affects of the competing reactions (e.g., H.sub.2 evolution and a related pH increase) especially at high cathode current densities. Though the electrical conductivity of such solutions can be improved by adding additional dissolved electrolytes (i.e., other than the metal sought to be deposited--e.g., acids) to each batch of water to be treated, such additional electrolytes: (1) require replacement; (2) are themselves often discharged into the environment or undesirably end up in the process water; and (3) do not significantly improve deposition current efficiency or deposition rate.
It would be desirable to improve such electrolytic metal scavenging/deposition process by means of a non-consumable ingredient which would improve deposition rates, deposition current efficiency (i.e., for lower energy consumption per pound of metal scavenged) and provide more uniform and higher quality metal deposits especially at the higher deposition rates.
It is an object of the present invention to provide an improved, more cost effective and energy efficient process for scavenging and depositing good quality metal from dilute aqueous solutions thereof at relatively high rates and current efficiencies.